[rubygems/rubygems] Avoid double CI runs in default Github Actions generated config

Otherwise CI will run once for every pull request update and once for
every push.

The disavantage is that people working on a non default branch without
creating pull requests won't get CI run at all.

I think the advantages are greater than the disadvantages, it's also my
personal workflow, and it's something desired enough so that someone
opened an issue about it.

So let's do it!

https://github.com/rubygems/rubygems/commit/924d916a73

Co-authored-by: NeimadTL <damientalbot26@gmail.com>

lib/bundler/templates/newgem/github/workflows/main.yml.tt

@@ -1,6 +1,10 @@
 name: Ruby
 
-on: [push,pull_request]
+on:
+  push:
+    - <%= config[:git_default_branch] %>
+
+  pull_request:
 
 jobs:
   build: